This invention was not developed in conjunction with any Federally sponsored contract.
Not applicable.
1. Field of the Invention
This invention relates to methods and devices for converting audio waves into electronic signals such as microphones, hydrophones, and sonic transducers.
2. Background of the Invention
Microphones are typically composed of a sensor element of some type which, when vibrated by incident sound or pressure waves, changes an electrical characteristic of the sensor element. For example, carbon microphones use a layer of carbon dust captured inside a flexible membrane. When the membrane is compressed or vibrated by sound waves, the electrical resistance through the carbon dust changes. By measuring the real time resistance of the dust (e.g. passing a current through the dust), an electrical signal which follows the sound signal is produced.
A xe2x80x9cdynamicxe2x80x9d microphone uses a coil surrounding a magnet, with the magnet suspended in the core of the coil by a flexible diaphragm. Incident sound waves on the diaphragm cause the magnet to move in the coil, which induces small amounts of electrical current into the coil. By amplifying these currents, an electrical signal which follows the sound signal is produced.
Yet another type of microphone is a xe2x80x9cribbon microphonexe2x80x9d in which a thin suspended conductive ribbon is placed in a magnetic field. As sound waves cause the ribbon to vibrate, an electrical current is induced proportional to the sound.
Condenser microphones are a form of a capacitor. One plate of the capacitor is a flexible diaphragm, and the other plate is stationary. As sound incident on the diaphragm causes the distance between the plates to change, the capacitance can be measured to generate an electrical signal representative of the sound waves.
Piezoelectric crystal microphones use an arrangement in which a crystal receives the sound waves, flexing or bending the crystal which generates an electrical charge proportional to the changes in pressure due to the sound waves.
All of these forms of pressure wave sensors, however, share a common factor in that they suspend their sensor element using mechanical means, often with a thin sheet of plastic stretched tight across an opening or cavity, and they all depend on flexing or bending of the sensor element in response to incident pressure waves. The characteristics of the mechanical suspension method (e.g. film over a round opening, film over an oval opening, etc.), the physical characteristics of the suspension material itself (e.g. thickness and elasticity of the film) determine, and the physical and electrical characteristics of the sensor material during flexing or bending give rise to response non-linearities, complex resonance phenomena, and loss of efficiency in wave-to-signal conversion.
For example, given certain dimensions, shapes and material characteristics, a particular sensor may be useful for a range of audible frequencies from 400 Hz to 4 kHz, but may not be responsive or useful for frequencies below 400 Hz (e.g. seismic measurements) or above 4 kHz (e.g. music and ultrasound). And, this particular combination of materials and shapes will often have complex harmonic characteristics and multiple resonant frequencies.
Therefore, there is a need in the art for a pressure wave sensor which minimizes or avoids mechanical and material losses due to mechanical suspension means employed, and which exhibits deterministic resonance and harmonic characteristics.